This invention relates to electrostatography and more specifically to method and means for processing charged image patterns. A charged image pattern which directly corresponds to an image is referred to as a latent electrostatic image. This latent electrostatic image may be stored on an insulator or a photoreceptor and may be formed either by placing a charged pattern on a dielectric as for example by writing with an electron gun in a vacuum or charging through a stencil or alternatively may be formed by classical xerographic techniques, i.e., charging a photoreceptor and selectively exposing same or by electroradiographic techniques including xeroradiography, ionography and electronradiography. Subsequent processing in these techniques commonly called development has heretofore been accomplished generally by deposition of pigmented marking material referred to as toner or developer employing conventional powder cloud, magnetic brush, cascade, or liquid development systems among others with or without a transfer process as required to provide an optical image in hard copy form on a suitable substrate when desired.
Generally speaking whatever development system is employed there are inherent problems experienced in loss of information, sensitivity limitations, image characteristics fixed per exposure, and image noise or granularity added by discrete nature of toner particles. These problems generally arise by virtue of the characteristics of the toner such as size, pigmentation and pigment-to-charge ratio and characteristics of the electrical and mechanical configuration of the development system.
More specifically in the well known enhancement of xerography there occurs partial loss of broad area information by supression in the development process. It is therefore found that development of the electrostatic image signal is generally less than complete, representing a loss in sensitivity in accordance with the limitations of the pigment deposited per charge neutralized, i.e., the pigment/charge ratio. For these systems in which many exposure photons are required to produce image charge equivalent to one toner particle, the image quality or signal-to-noise ratio and/or sensitivity is decreased which necessitates higher exposure. Finally, toner development of the image generally involves loss or degradation of the latent charge image so that development of an alternate hard copy optical image via a different development system characteristics may involve another exposure and latent charge image generation step.
In xeroradiographic imaging applications an X-ray image is formed on a selenium alloy photoreceptor by exposure of X-ray radiation through a object onto said photoreceptor, said charged photoreceptor resulting in an electrostatic charge pattern. This latent image is developed directly on the photoreceptor employing powder cloud development and is then transferred to paper, fused, and results in an edge enhanced image on a hard copy for a physican to diagnose. When degradation of the latent image information occurs during development, transfer, and/or fusing, obvious concerns arise with regard to the loss of medical diagnostic information. Furthermore, it is found in these processes that photoreceptor life is limited by the repeated brush cleaning of the untransferred developer or toner. It should be noted, in addition, that this type of X-ray technique is also used in various and sundry commercial applications such as discerning defects in casting wells or the like, etc. Thus, although powder cloud development is an inexpensive, effecient method of providing edge enhanced images it is found not to provide emphasis of density ranges and therefore to have a limited latitude in usage.
Thus it is an object of this invention to provide an imaging system which is devoid of the above noted deficiencies.
Still another object of this invention is to provide an imaging system which preserves the integrity of the latent electrostatic image.
Yet another object of this invention is to provide a novel imaging system.
Again another object of this invention is to provide an imaging system which processes the electrostatic charge pattern so that it may be effectively utilized in desired applications.
Yet still another object of this invention is to provide an imaging system which eliminates conventional development systems and their inherent disadvantages.
It is a further object of this invention to provide an imaging system which effectively preserves, utilizes and processes electrostatic charge patterns.
Still another object of this invention is to provide a novel imaging system which may store a charged electrostatic charge pattern, retrieve it and transmit it as desired.
Another object of this invention is to provide a system to which conventional image processing can be applied.
These and other objects of the instant invention are accomplished, generally speaking, by providing an imaging system wherein a latent electrostatic charge image is provided, e.g., on a photoconductor or on an insulating surface, capable of bearing a charge image, the image is then scanned with an electrometer type device to detect the magnitude of the induced charge at each point on the surface, the resulting electrical signal is transmitted through appropriate devices so that the signal may be processed as desired to produce visible images on video monitors or on hard copy devices. These images may exhibit contrast stretching, magnification, pseudo color or other typical image processing features. Further, the image information in electrical form may be transmitted to remote locations or stored in suitable memories for later utilization.
More specifically a charge pattern is formed on a dielectric or photoconductor as desired as, for example, in electrophotographic or xeroradiographic applications where a photoconductor is uniformly charged and then selectively exposed to provide the resultant latent electrostatic image which is then captured by any suitable means, for example, by employing either a single wire microelectrometer or a multiple wire microelectrometer which senses, measures, and transmits accurately the electrostatic pattern to a processing media. The term "capture" hereinabove recited describes the process of sensing and measuring the magnitude of an induced charge as described above.
This charge capture of the latent electrostatic image is accomplished with minimal noise added to the signal-to-noise ratio inherent in the latent electrostatic charge image so that it presents several alternatives for processing. The charge image may be read as analog signals to a video screen processed as such with a suitable contrast gain applied as a function of the signal level to provide a video image of selectable brightness and contrast. Alternatively, the analog signal can be used to generate a hard copy using a suitable device.
Secondly, the charge image can be stored in analog or digital form by various means and processed in various methods including high or low spatial frequency suppression, contrast stretching, clean-up of structured "noise" (not statistical noise), and then displayed on video or made into hard copy. In this case, once stored, the captured latent charge image signals can be alternately processed in different ways to provide several images of different characteristics as desired from the single original exposure.
In addition, any types of operations with image information as presently accomplished, pursued or proposed which are captured by optical methods could be used with this approach with the advantage of not having degraded the latent electrostatic image by a prior conventional development.
Further computer analysis of image information can be readily made, for example, via Fourier or textural analyses to detect or suppress certain size or structure image formation.
This approach can produce, from a single exposure, multiple images each of which has a distinctly different character. The multiple images may emphasize and display different types of information more effectively than a single image.
For example, in radiography, images exhibiting high spatial frequencies better display fine structure such as small calcifications in tissue, whereas images exhibiting low spatial frequencies better display broad area structure such as metastatic bone cancer. This can presently be done by connecting the image information in a film radiograph or a xeroradiograph to an electrical signal by optical scanning. However, whatever development system is used to obtain the radiograph imparts a quality to the image which limits subsequent processing as hereinbefore described.
More specifically in employing a scanning electrometer system, for example, for capturing and displaying latent electrostatic image information the following advantages are realized:
(1) The image is not degraded by development nonuniformities, edge enhancement, transfer, or fusing as, for example, in conventional xerographic development systems.
(2) Physical contact with the photoreceptor is eliminated and accordingly photoreceptor life is increased.
(3) The system can operate effectively at very low X-ray exposures.
(4) The system provides for the selection of the type and amount of edge enhancement during processing that may be desired to most effectively display the image information.
(5) Various image features may be emphasized by adjusting the density and contrast of the displayed image to emphasize the information or diagnosis desired, especially in medical diagnostic techniques.
(6) From single X-ray exposure, multiple images can be obtained exhibiting different characteristics to best display the different types of information. Thus, more information is readily extracted with minimal exposure of the patient to radiation.
(7) Pseudo color can be added for emphasis of image details in accordance with conventional techniques.
(8) Other techniques of image processing could be applied to the instant invention by those skilled in the art.
One embodiment of the instant invention uses a single wire microelectrometer. This consists of a wire with cross section area of approximately the same size as the smallest picture element to be resolved. The wire is connected to a field effect transistor which has high input resistance. The output of the field effect transistor is connected to an amplifier to further amplify the signal. When the wire of the microelectrometer is placed in close proximity to the latent charge image, a charge is induced on the wire with a magnitude which is proportional to the charge in the picture element. In turn, the output of the amplifier is proportional to the induced charge. To capture an image, the microelectrometer is scanned across the image bearing surface. The result is a time varying signal from the amplifier which represents the spatial variation of charge along the line of the scan. By repeated scanning in this fashion, the entire two dimensional image is captured and transformed into electrical signals.
The magnitude of the induced signal on the microelectrometer wire is determined, not only by the charge in the picture element, but also by the capacitance between the picture element and the wire.
The gap capacitance is a function of the shape of the wire and the distance between the wire and the surface. Therefore, it is important to either maintain a precise spacing between the wire and the surface or to measure the spacing at each point and make appropriate corrections to the signal amplitude. For example, the gap may be maintained with an air bearing providing the undulations in the image bearing surface are not too large or abrupt. Alternatively, the electrometer may be rigidly fixed and the distance sensed by optical techniques, etc.
Care should be taken to design the electronic circuitry to control leakage of charge from the probe to ground to avoid adding noise to the system. Care should also be taken to design the circuit and choose components to minimize fluctuation in performance with temperature variation.
Photoreceptors are subject to dark decay. Since the scan requires a finite length of time, the photoreceptor material should be selected to minimize dark decay so that the signal truly represents the latent electrostatic image. In the event that dark decay is not acceptable at desirable scan rates, this deleterious effect can be mitigated by providing means to measure the dark decay and compensate electronically.
In another embodiment of the system of the instant invention, a multiple probe electrometer array is employed comprising a number of single probe electrometers which are mechanically coupled such that more than one path of readings are taken with one scan across the latent image. The more electrometers which are employed, the less number of scans are required to do a complete raster scan. Increasing the number of probes increases the electronics and gap maintenance complexity while it reduces the mechanics, image interlace complexities and processing time.